In electrostatographic applications, a charge retentive surface (e.g. photoconductor, photoreceptor, or imaging surface) is electrostatically charged, and exposed to a light pattern of an original image to be reproduced to selectively discharge the surface in accordance therewith. The resulting pattern of charged and discharged areas on that surface form an electrostatic charge pattern (an electrostatic latent image) conforming to the original image. The latent image is developed by contacting it with a finely divided electrostatically attractable powder referred to as “toner.” Toner is held on the image areas by the electrostatic charge on the surface. Thus, a toner image is produced in conformity with a light image of the original being reproduced. The toner image may then be transferred to a substrate (e.g., paper), and the image affixed thereto to form a permanent record of the image to be reproduced.
Frequently, residual toner particles adhere to the photoconductive surface after the transfer of the developed image to the copy sheet. These residual toner particles may be “right sign toner,” i.e. toner particles charged to a polarity which attracts the toner particle to the latent image, or “wrong sign toner,” i.e. toner particles charged to a polarity which repels the toner particle from the latent image.
A cleaning subsystem is commonly used to remove the residual toner particles from the photoconductive member. The cleaning subsystem typically includes one or more rotating cleaning brushes. Brush cleaners operate by removing the toner from the photoreceptor both with mechanical and/or electrostatic forces. The fibers on the brush touch the untransferred toner and the toner is removed from the photoreceptor onto the brush. The toner on the brush is then transported to a detoning device (e.g. flicker bar, detoning roll, air system, combs, etc.) removing the toner from the brush (i.e. detoned).
Electrostatic brush (ESB) cleaners are designed to clean right and wrong sign toner from the photoreceptor as it passes through the cleaner. Conventional electrostatic brush cleaners consist of two or more brushes electrically biased to remove toner and other debris from the photoreceptor surface. Prior to encountering the brushes, a preclean charge device adjusts the charge of the incoming toner to the natural tribo charging polarity of the toner. The first brushes are biased opposite to the polarity of the right sign toner so that this toner is removed. The last cleaning brush is biased opposite to the first brushes so that the wrong sign toner is removed. ESB cleaners typically include a housing for rotatably mounting the cleaning brushes. The housing may include an air manifold or vacuum for removing the toner from the brushes.
The brushes of an ESB cleaner are typically driven by a variable speed, pulse-width-modulated (PWM), D.C. servo motor. The cleaner servo control is commanded to turn the cleaning brushes at a predetermined velocity. The controller generates a PWM (Pulse Width Modulated) control signal to command the motor power. The typical operating range of a cleaner motor is between 55-70% of the PWM signal maximum. The speed of the cleaner servo motor may be measured with a shaft encoder that generates an electrical signal that corresponds to the rotational speed of the motor. Various factors, such as, for example, a change in the load on the cleaner motor, may cause a change in the velocity of the motor. A servo controller samples the encoder pulse output, and if the servo controller determines that the cleaner motor is operating above or below a currently commanded set point for the motor velocity, the servo controller calculates a new PWM duty cycle for the next period to adjust the cleaner servo motor velocity to the commanded velocity.
Previously known cleaning subsystems may be configured to detect when a cleaning subsystem is not working within design limits indicating that a catastrophic stress condition may have occurred within the cleaning subsystem. Typically, an indication that a cleaning subsystem is not working within design limits may be a PWM signal for controlling the cleaning motor that has reached its maximum or minimum limits. A catastrophic stress condition that may cause an adjustment of the PWM signal to its maximum or minimum limits may comprise a motor or cleaning system failure that prevents the motor from rotating the cleaning brushes at the commanded speed indicated by the PWM signal. For example, a cleaning motor or system failure may prevent the motor from rotating the cleaning brushes or may cause a dramatic increase in the load on the motor resulting in an adjustment of the PWM signal to its maximum limit (e.g. 100% duty cycle). Similarly, a cleaning motor or system failure may result in a dramatic decrease in the load on the motor such as if a cleaning brush comes uncoupled from the motor. A dramatic decrease in the load on the motor may cause an adjustment of the duty cycle of the PWM to its minimum limit (e.g. 0% duty cycle). Previously known cleaning subsystems may be configured to disable the cleaner motor and generate an alert signal for the main control system when the subsystem detects such a catastrophic stress condition.
A stress condition may occur, however, that does not cause the cleaning subsystem to work outside of the design limits for the system and may, thus, go undetected by previously known cleaning subsystems. One such non-catastrophic stress condition that may occur is an image substrate, such as a sheet of paper, remaining in contact with the photoreceptor after the transfer station so the sheet enters the cleaner housing. If this condition is not detected, the paper may get wrapped around the rotating brushes and prevent the brushes from cleaning the residual toner from the photoreceptive surface. Paper stuck in the cleaner housing may also rub against the photoreceptor and scratch the photoreceptive surface. Additionally, paper in the cleaner housing may increase the load on the cleaner servo motor driving the brushes making malfunctions of the cleaner subsystem more likely.
To minimize the chances of an image substrate getting into the cleaner housing, imaging devices have frequently contained various types of devices and techniques to strip sheets from the photoreceptor surface. For example, stripper fingers may be placed adjacent the photoreceptor to mechanically strip a sheet tacked to an image support surface before it enters the cleaner. These devices, however, are more effective with heavy stock paper than they are with light weight media. Additionally, the constant scraping action between the photoreceptor and the stripper fingers may, over time, damage the photoreceptor surface.
Another non-catastrophic stress condition that may occur at the cleaner subsystem is a photoreceptor “suck up” condition. This condition arises when the cleaner housing gets too close to the photoreceptor and the air manifold of the housing draws the photoreceptor into the housing. Consequently, the cleaner brushes are pressed against the photoreceptor surface thereby causing friction on the brushes which increases the load on the cleaner servo motor. Again, the brushes and/or housing contacting the surface of the photoreceptor may scratch and permanently damage the photoreceptor.
When a non-catastrophic stress condition occurs, residual toner particles may not be properly cleaned from the photoreceptive member. Because non-catastrophic stress conditions are typically not detected by previously known cleaning subsystems, print defects may occur in subsequent print jobs due to the residual toner particles remaining on the photoreceptor. Additionally, undetected stress conditions on the cleaning subsystem may cause damage to the photoreceptive surface, such as scratches, which may lead to streaks on the prints. Undetected stress conditions may be especially problematic during long print runs where the operator may leave the imaging device to do other tasks. As a consequence, an entire print run may be contaminated.